This invention relates generally to digital optical disks and more specifically to erasable and rewritable digital optical disks.
In general, there are three types of digital optical disks: read-only, recordable (also called Write-Once or Write-Once-Read-Many (WORM)) and erasable (also called rewritable). Examples of commercially available read-only optical disk technologies are the Compact Disk (CD) for digital audio and the Compact Disk-Read Only Memory (CD-ROM) for computer data. Compact Disk-Recordable (CD-R) drives and media are also commercially available. An example of an erasable (rewritable) optical disk technology is the Magneto-Optic disk, widely used for computer data storage.
For any electromechanical disk technology, whether magnetic or optical, there are physical limitations on information density. Some of these limitations are mechanical, for example, the tolerance on angular velocity for a disk rotation control system, or the tolerance on positioning a transducer relative to a data track. Other limitations are imposed by transducer or media physics, for example, diffraction effects in optics. In some technologies, for example many computer magnetic disks, a disk remains in a single drive for writing and reading. If the same drive writes and reads the disk, some repeatable mechanical effects may be ignored. For example, if the angular velocity of the disk is at the high extreme of an allowable range, this is not a problem since a disk recorded at the high speed will be read at the same high speed, so that data writing rates and data reading rates are identical. If a disk must be interchanged (written in one drive and read in a different drive), then the various worse case limitations of both drives combine, and the tolerances for each drive must be narrowed in order to meet an overall specification with interchange. For mass-produced read-only media, for example CD-ROM""s, the process for writing a metalized master disk for molding of copies is typically a very high precision (and expensive) process. Very little of the overall allowable tolerances are used by the writing process, enabling most of the overall allowable tolerances to be used in the reading drives. Therefore, for example, portable read-only CD-ROM drives for notebook computers can have substantially broader tolerances (and substantially lower cost) than the drives producing the master disks for molding. However, if each of the drives involved in interchange can write data to a disk, then each of the drives must be limited to no more than half of any particular overall tolerance specification. For example, if the angular velocity at any particular radial head position must be accurate to xc2x11.0%, the angular velocity for each drive involved in interchange must be accurate to xc2x10.5%.
Some data formats assume that an entire medium will be recorded at one time. Other data formats assume that individual random sectors of data can be erased and rewritten. Typically, format provisions for rewriting individual sectors reduce the overall effective data capacity. CD, CD-ROM, and CD-R formats are designed for maximum data capacity with no provision for rewriting individual sectors. CD-ROM""s are organized into data sectors and each data sector must be phase synchronized with the preceding sector and with the following sector. As a result, compatible CD-R disks must be written in sequence, starting from the first sector and writing each sector in the physical order that they appear on the disk. The standard CD format specifications do not support the ability to write or overwrite individual sectors with random access or to append to a partially recorded medium. Typically, recordable drives that can append to a partially recorded medium, and drives that can erase and overwrite previously recorded data, must provide data gaps for accommodating angular speed variations between drives and must provide additional clock synchronization patterns for accommodating clock differences between drives. For example, magnetic disks and magneto-optic disks are typically formatted into sectors, with each sector including a preamble for synchronizing a write clock, and with each sector including extra space at the end to allow for variations in angular velocity, each of which reduces effective data capacity. CD, CD-ROM, and CD-R formats do not have sector preambles for synchronization or extra space at the ends of sectors. In general, drives that can append or rewrite individual sectors with random access have a reduced effective disk capacity relative to drives, with the same bit density, that write an entire disk at one time.
In addition to clocking precision requirements and angular velocity precision requirements, writing drives must meet precision requirements for radial position or track following. Writable and rewritable optical disk media often have a predefined track, typically a land and groove structure. Other approaches to predefined tracks may be found in U.S. Pat. Nos. 5,213,859 and 5,204,852. Typically, for drives using grooves or similar approaches, the bandwidth and signal to noise ratio for track centering of the writing laser are not as good as that obtained by the high precision drives used for mastering read-only media. In addition, some servo approaches proposed for writable media may be incompatible with read-only formats.
Various digital optical disk standards are being planned in advance of available technology. That is, various capacities and formats have been proposed for future standardization, even though corresponding drives and media may not yet be available or practical. An example is erasable (rewritable) Digital Versatile Disks (DVD) (previously called Digital Video Disks). The proposed standards for erasable DVD""s assume that the model established by CD""s will continue. That is, the proposed standards assume that at any particular bit density, read-only and recordable (write-once) media will have the highest possible data capacity and erasable (rewritable) media will have a reduced effective data capacity. The proposed standards assume that erasable (rewritable) systems must have a lower capacity than read-only and recordable (write-once) systems due to the extra overhead for synchronization and gaps for drive speed variation. The proposed standards assume that writable media must have a land and groove structure or other predefined track servo information. In general, format differences between proposed erasable disks and read-only disks are incompatible, so that drives must be designed to read two separate formats, or drives designed only for read-only and write-once disks will not be able to read erasable disks. The proposed evolution or xe2x80x9cmigration pathxe2x80x9d typically specifies that for each new step in bit density, there will first be read-only products that extract the maximum possible capacity from the anticipated technology (shorter wavelength lasers and improved media characteristics), followed by recordable (write-once) products having the same capacity as the read-only products, followed by erasable products with the same bit density as read-only and recordable products, but with a lower effective capacity.
There is a need for erasable (rewritable) optical disks having the same format and the same effective capacity as read-only and write-once disks.
A reference clock track and optional additional prerecorded phase synchronization patters are provided to enable writing of any random sector with frequency and phase matching to the preceding and following sectors. The reference clock track and other phase synchronization patterns eliminate the need for preambles and extra space (gaps) for speed variation.
In a first embodiment, a disk has multiple layers, with at least one data layer and a reference surface. A spiral track on the reference surface has prerecorded patterns to be used for clocking. In a variation of first embodiment, the reference surface is also used for radial tracking control, eliminated the need for predefined tracks in the data layers. The reference surface is produced using the same technology as for read-only media, and is therefore very precise, low cost, and permanent. An additional laser system may be required to read the reference surface. The data layers may be unpatterned prior to writing. Alternatively, the data layers may include embossed sector or block headers to augment clock phase precision.
In a second example embodiment, a single circular permanent (non-erasable) clock track is provided on the erasable medium. The disk is then divided into radial zones, so that within each zone, the angular velocity of the disk is constant. A clock signal from the permanent clock track is then ratioed for each radial zone. An additional laser system is required to read the clock track. However, in many drives, a second laser will be required for backwards compatibility. The same second laser may be used for the clock track. The two embodiments are not mutually exclusive and instead can be combined. As in the first embodiment, the rewritable data area in the second embodiment may include embossed sector or block headers to augment clock phase precision. Also, zones with constant angular velocity could be implemented in the first embodiment.
Each of the example embodiments provides the following advantages:
1. The format and effective capacity of an erasable (rewritable) disk is identical to the format and effective capacity of read-only and recordable (write-once) disks. Either of the example embodiments may also be implemented with a write-once medium, enabling partial writing with later appending.
2. Once written, the disk of either sample embodiment can be read in a standard read-only player. The reference surface (or clock track) and the additional laser system (if required) are used only during writing.
3. The incremental cost of adding a reference surface or a clock track is nominal.
The first embodiment (reference surface) provides the highest clock precision of the two embodiments and also provides radial tracking information. The second embodiment (separate clock track) provides simplicity, lower cost, and improved random access times. The second embodiment is simpler and may have lower cost because no additional layers are added to the disk and because a second reading laser may be present anyway for other reasons. Zones with constant rotational speed, in either embodiment, may provide improved random access times.